A pneumatic brake booster for a motor vehicle generally comprises a casing with a wall made of sheet metal. The casing comprises a variable-volume front chamber, separated from a rear chamber, also of variable volume, by a partition. The partition is formed of a sealed and flexible diaphragm and a rigid skirt plate. A master cylinder partially penetrates the front chamber of the casing, by means of a penetration snout. The front chamber, placed on the master cylinder side, is pneumatically connected to a source of vacuum. The rear chamber, on the opposite side to the front chamber and situated on the brake pedal side, is connected pneumatically, in a way controlled by a valve, to a source of driving fluid.
At rest, that is to say when a driver is not depressing the brake pedal, the front and rear chambers are connected to one another. The rear chamber is isolated from atmospheric pressure. Under braking, the front chamber is first of all isolated from the rear chamber, then air is let into the rear chamber. This admission of air has the effect of driving the partition forwards. The rigid skirt then drives a pneumatic piston of the booster which itself drives a primary piston of the master cylinder via a push rod. A pulling-out force that may exceed 4000 Newtons is then exerted on the master cylinder. It is absolutely essential for the master cylinder to be secured to the casing of the booster in order to prevent any movement of the master cylinder with respect to the casing.
The casing has the overall shape of a cylinder. It is closed at its two ends by a cover and a back wall respectively. The back wall of the casing is not flat. It is domed. Initially, booster casings were manufactured with a back wall of flat circular overall shape. It was then found that if the flat internal face of the back wall of the casing had the overall shape of a diamond, the metal sheet of the cylinder of the casing was better able to withstand the deformations due to the forces involved during braking. Since then, use has therefore been made of a cylinder having a diamond-shaped flat face. An inclined face extends from the edge of the flat face towards a cylindrical edge of the casing.
Generally, the master cylinder is fixed to the casing via a flange of the master cylinder. The flange, which extends at right angles to a master cylinder body, is pressed against the external face of the cylinder of the casing. The connection between the flange and the cylinder of the casing is achieved using a set of fixing screws. At least one screw, and generally two, passes through the cylinder and the master cylinder flange. A screw head is situated inside the casing. The screw is held in position by a nut.
Usually, a screw reinforcement stiffens the attachment region. The reinforcement is interpolated between the screw head and the flat face of the cylinder of the casing. The reinforcement has a screw orifice. This reinforcement advantageously has a flat face in contact with the flat face of the cylinder of the casing, and an inclined face in contact with the inclined face of the cylinder. It is therefore preferable for the inclined face of the reinforcement to have a slope equal to the slope of the inclined face of the wall. This then provides contact between the reinforcement and the internal faces, flat and inclined respectively, of the cylinder.
If the placement of the reinforcement is automated, the placement machine needs to be set up very precisely. Likewise, if placement is performed by hand, the operator needs to pay special attention. This is because the reinforcement needs to be positioned at a location that allows it to reinforce precisely that region of the sheet metal cylinder that is most heavily stressed under working conditions. The slightest offset of the reinforcement with respect to the internal face of the cylinder means that the slopes of the reinforcement no longer coincide with those of the cylinder. The regions subjected to the pulling-out forces are therefore not correctly reinforced. The sheet metal of the cylinder of the casing will experience excessive work under braking, and will weaken. This weakening leads to yielding of the sheet metal at the location of the fixing region, and to the master cylinder becoming detached.
Once the reinforcement has been correctly located, the screw is introduced into the screw orifice. The screw is tightened using a nut. This tightening step also requires great attention, in order to avoid any rotation of the reinforcement which could lead to incorrect positioning of the said reinforcement.
The current system for attaching the master cylinder to the booster therefore requires great precision, and repeated checks. This results in a significant loss of time. This loss of time has an impact on the cost of manufacture of the booster.
Furthermore, in such a device, use is generally made of two fixing screws. The screws are arranged at the furthermost ends of the diamond-shaped flat internal face. The points of contact between the master cylinder and the casing are located only at the fixing screw heads and around the perimeter of the reinforcements. The stresses exerted on the master cylinder are therefore concentrated in these attachment regions. It is necessary for the sheet metal of the cylinder to be fairly thick, and to this thickness is added the thickness of the reinforcements, in order to be able to compensate for this poor distribution of stresses.
Finally, the number of parts needed for attachment (screws, reinforcements, nuts) in part explains the high cost of manufacture of a booster.